1. Field of the Invention
The present invention relates to a fabrication process for electrodes used with electric double-layer capacitors (EDLCs), lithium ion secondary batteries, etc., and an electrochemical device fabrication process that involves part of that process.
2. Description of the Prior Art
Electrochemical devices such as electric double-layer capacitors (EDLCs) and lithium ion secondary batteries are now widely used for cell phones, PDAs (personal digital assistants), etc.
Electrodes for such electrochemical devices are fabricated by coating a collector (support carrier) such as an aluminum or copper foil with an electrode-formation coating material comprising an active substance, a binder, a binder soluble solvent or a solvent for imparting plasticity to an electrode, which is applied when an insoluble binder is used (these solvents are collectively called the “binder solvent”), and an optionally used conductive aid such as carbon black.
When polyvinylidene fluoride (PVDF) is used as the binder for the preparation of the electrode-formation coating material, N-methyl-2-pyrrolidinone (NMP) is usually used as the binder solvent.
In an electrode fabrication process by such coating film formation, the binder solvent remains in the electrode film in no small amounts. Especially with the fabrication of an electrochemical device with which activated charcoal having a large surface area is used as its active substance, there is a problem that the electrostatic capacity of the activated charcoal dwindles due to the adsorption of the binder solvent onto its surface. Also, the remaining binder solvent is responsible for drops of the durability and reliability of the electrochemical device.
To solve such problems, there have been various techniques proposed so far in the art, for instance, an electrochemical cell control method using a supercritical state carbon dioxide (Domestic Republication of PTC 2000-506669), a secondary battery fabrication process in which an electrode-formation material is washed with a supercritical state fluid before or after electrode formation (JP-A61-68870), and a fabrication process for activated charcoal for electric double-layer capacitors which involves treatment with water in a supercritical or near state (JP-A2004-31713).
However, the binder solvent used for the fabrication of an electrode for electrochemical devices such as electric double-layer capacitors (EDLCs) and lithium ion secondary batteries still remains in the electrode in no small amounts only by virtue of ordinary drying techniques.
Further, in activated charcoal or other porous carbon material used as the active substance, there are surface pores. The pores are broken down into macro-pores (of 50 nm or greater in diameter), meso-pores (of 2 to 50 nm in diameter), and micro-pores (2 nm or less in diameter).
The binder solvent used on electrode fabrication is difficult to remove, because of adsorption to such pores. In particular, it is very difficult to remove the solvent adsorbed to the micro-pores.
A certain solvent polymerizes upon heating. As that solvent is heated while adsorbed onto a pore, it causes the solvent to polymerize within the pore; solvent removal by heating may possibly clog up the pore with the polymer.
Thus, as the solvent is adsorbed to the surface pores of activated charcoal or the pores are clogged up with the polymer resulting from the solvent, electrolyte ions fail to have access to the surface of activated charcoal, offering a problem that the electrostatic capacity of activated charcoal dwindles.
With such considerations in mind, it is still desired to make further improvements in the above prior art techniques, thereby achieving a process for the fabrication of an electrode for electrochemical devices, which ensures that the electrostatic capacity of a carbonaceous material as an active substance is increased with much improvement in the reliability of an electrochemical device.